1. Field of the Invention
The present invention relates to a new process for the preparation of polyisocyanates containing isocyanurate groups by the catalytic trimerization of monomeric polyisocyanates using novel trimerization catalysts and to their use as the isocyanate component for the preparation of polyisocyanate polyaddition products, preferably polyurethanes.
2. Description of the Prior Art
Numerous catalysts for the trimerization of organic isocyanates, in particular polyisocyanates, are known (J. H. Saunders and K. C. Frisch, Polyurethanes Chemistry and Technology, page 94 et seq (1962)). Strong organic bases are suitable as trimerization catalysts, e.g. carboxylic acid metal salts which are alkaline in nature, metal alcoholates, metal phenolates, alkali metal carbonates and tertiary amines.
The catalysts are frequently used in combination with one another or together with cocatalysts such as mono-N-substituted carbamic acid esters (A. Farkas and G. A. Mills, Advances in Catalysis, Vol. 13, 393 (1962)).
In the more recent state of the art processes, the trimerization catalysts used are special organic bases some of which are required to be prepared by elaborate methods of synthesis.
Thus, for example, the trimerization of aromatic polyisocyanates is catalyzed with Mannich bases (DE-OS No. 2,551,634 and DE-OS No. 2,641,380) or tertiary phosphines. When phosphines are used, uretdiones are first formed and then converted to the isocyanurate in a second reaction phase (DE-OS No. 1,201,992). The trimerization of (cyclo)aliphatic diisocyanates has in recent times frequently been catalyzed with organic bases having a betaine structure such as quaternary ammonium hydroxides (EP-A No. 010,589 and EP-A No. 009,694): aminimides (J. E. Kresta, R. J. Chang, S. Kathiriya and K. C. Frisch, Makromol. Chem. 180, 1081 (1979)): and aziridine derivatives in combination with tertiary amines (DE-OS No. 2,325,826).
The use of the quaternary ammonium salts obtained from tertiary amines and alkylating esters of acids of phosphorus as trimerization catalysts is described in DE-OS No. 3,227,489.
The present invention provides a new class of valuable trimerization catalysts which are distinguished by the combination of numerous remarkable advantages:
1. The new catalysts are suitable for trimerizing both aromatic and aliphatic polyisocyanates. PA0 2. Both the new catalysts and the products of their reaction with the catalyst poisons described below are soluble in the starting materials and end products of the process according to the invention so that elaborate procedures for separation are obviated. PA0 3. Trimerization with the aid of the new catalysts may be carried out either solvent-free or in the presence of solvents at comparatively low temperatures so that pale yellow, clear, low viscosity polyisocyanates containing isocyanurate groups are obtained as products of the process. PA0 4. The slightly exothermic trimerization reaction can be safely and easily controlled both when carried out continuously and when carried out batchwise. PA0 5. The catalysts can be easily and inexpensively prepared and are almost odorless.